The present invention relates to a ceramic wiring substrate.
Recently, a multilayer wiring substrate has been put into practical use for high performance of a computer. In the multilayer wiring substrate, wiring is multilayered, wiring lengths between LSI elements such as a logic gate or a memory mounted to the substrate and between the LSI elements and a power source and a ground electrode are shortened to reduce delay time of transfer of a signal, thereby obtaining a high speed operation of the computer.
In the multilayer substrate, the lower the permittivity of a substrate material, the better. The reason for this is as follows. In order to constitute a memory module which packages many memory chips, many memory chips must be connected in parallel, so that signal lines are elongated. As a result, a capacitance relative to ground of a signal line formed between the signal line and a ground electrode is increased. As a result, a load capacitance of a drive element for driving a memory element is increased, and a rise time of a signal is increased, thereby adversely affecting an access time. For this reason, a portion which forms a signal line of the substrate preferably has a low permittivity, and efforts have been made to obtain a low permittivity of the substrate material. However, to obtain a low permittivity is not easy because the permittivity of the substrate conflicts with mechanical characteristics such as strength and thermal characteristics such as a sintering temperature or a thermal expansion coefficient.
A method of obtaining a multilayer substrate with a low permittivity by a low sintering temperature is disclosed in an article entitled "Low Firing Temperature Multilayer Glass-Ceramic Substrate" which is announced in IFFE TRANSACTION ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. CHMT-6, No. 4 DECEMBER 1983 pp. 382-388. According to this method, a multilayer wiring substrate is realized using glass-ceramic which can be sintered at a low temperature of 900.degree. C. and has a permittivity of 7.5.
However, the permittivity of the above value is insufficient.